Technical Field
The present invention relates to a method for manufacturing a rare-earth magnet.
Background Art
Rare-earth magnets containing rare-earth elements such as lanthanoide are called permanent magnets as well, and are used for motors making up a hard disk and a MRI as well as for driving motors for hybrid vehicles, electric vehicles and the like.
Indexes for magnet performance of such rare-earth magnets include remanence (residual flux density) and a coercive force. Meanwhile, as the amount of heat generated at a motor increases because of the trend to more compact motors and higher current density, rare-earth magnets included in the motors also are required to have improved heat resistance, and one of important research challenges in the relating technical field is how to keep magnetic characteristics of a magnet operating at high temperatures.
Rare-earth magnets include typical sintered magnets including crystalline grains (main phase) of about 3 to 5 μm in scale making up the structure and nano-crystalline magnets including finer crystalline grains of about 50 nm to 300 nm in nano-scale. Among them, nano-crystalline magnets capable of decreasing the amount of expensive heavy rare-earth elements to be added or not including such heavy rare-earth elements added while making the crystalline grains finer attract attention currently.
The following briefly describes one example of the method for manufacturing a rare-earth magnet. For instance, in a typical method, Nd—Fe—B molten metal is solidified rapidly to be fine powder (magnetic powder), while pressing-forming the fine powder to be a sintered body. Hot deformation processing is then performed to this sintered body to give magnetic anisotropy thereto to prepare a rare-earth magnet (orientational magnet). The hot deformation processing is performed by extrusion such as backward extrusion or forward extrusion, or upsetting (forging), for example.
Meanwhile it is known that, in each step of such a manufacturing process including the preparation and conveyance of magnetic powder, the preparation of a sintered body and the preparation of a rare-earth magnet, a product in process may come into contact with the air (oxygen thereof), and so the oxygen density in the composition of the product in process may increase or the product in process may be oxidized, and the final rare-earth magnet may have degraded magnetic performance, such as in coercive force. For instance, it is known that, during the hot deformation processing, oxygen contained in a magnet material destroys the Nd—Fe—B main phase, which becomes a factor to decrease the residual flux density and the coercive force. It is further known that, during grain-boundary diffusion of a modified alloy to recover the coercive force after hot deformation processing as well, oxygen left inside becomes a factor to inhibit the modifier alloy from permeating through the inside. It is also known that oxygen taken in a magnet reacts with a rare-earth element in the grain-boundary phase to form an oxide, and so the component in the grain-boundary phase that is effective to separate the main phase magnetically decreases, resulting in a decrease in coercive force of the rare-earth magnet.
To avoid these problems, a technique to avoid a contact with oxygen in the manufacturing process of a rare-earth magnet or to decrease the oxygen density has been proposed and been put to practical use.
For instance, Patent Documents 1, 2 disclose a technique of storing magnetic powder for rare-earth magnet in an airtight vessel filled with inert gas, and performing sintering while supplying powder from this vessel to a mold.
Patent Document 3 discloses a method for manufacturing a rare-earth magnet, in which magnetic powder for rare-earth magnet is charged in a metal can, followed by hermetical-sealing while evacuating, and then hot extrusion pressing is performed by heating this can to manufacture a rare-earth magnet.
Patent Document 4 then discloses a method for manufacturing a rare-earth magnet of surrounding a rare-earth magnet ingot with a metal material for hermetically-sealing, followed by hot processing.
According to the techniques disclosed in these Patent Documents, the density of oxygen that comes into contact with magnetic powder, a sintered body and the like during the manufacturing process of a rare-earth magnet can be reduced.
The manufacturing methods disclosed in Patent Documents 1, 2, however, include the step of charging magnetic powder into a mold from an airtight vessel, and so its workability is not good. Additionally, these methods are time-consuming and the cost is required to prepare a vessel, and so the manufacturing cost will increase.
In the manufacturing methods disclosed in Patent Documents 3 and 4, a metal can, for example is hot-pressed. Herein, since Nd—Fe—B magnetic powder for rare-earth magnet tends to be oxidized more than general metal, the magnetic powder inside of the metal can is easily oxidized prior to oxidation of the metal can, for example. In this way, a large effect to suppress oxidation of metal powder cannot be expected.